AGL needs to unify the use of IMAGE_FSTYPES to minimize the footprint and standardize the output. This minimizes the download/mirror needs and streamlines the documentation.
E.g.:

• by default we build the ext4.xz
• plus the netboot ext4.gz.u-boot
• optional the BSP defines the sdcard image (but that should be WIC, thus the CANNED_WKS mechanism applies) or hddimage/wic (intel)

Other filesystems or output formats need to go through SAT.